1. Technical Field
The present invention relates to an image data compression device, an encoder, electronic equipment and a method of compressing image data.
2. Related Art
As a general-purpose encoding method for multimedia data such as a static or motion image data and an audio data, Moving Picture Experts Group Phase 4 (MPEG-4) is standardized. Recent mobile devices can encode and decode image data in conformity to this MPEG-4 standard, and they can play a video and send and receive data through networks.
In the MPEG-4 standard, a compressed motion image data, which is made by encoding the image data, should be produced at a regular rate. However, compression efficiency widely fluctuates based on a content of the image data when the motion image data is compressed. MPEG-4 Visual Part (recommendation report ISO/IEC 14496-2: 1999(E) Annex L) discloses a rate control method with which the compressed data is produced at the regular rate by controlling an amount of generated codes in order to keep the fluctuation to a certain level.
When the encoding (compressing) process of the MPEG-4 is conducted, all the process may be carried out by hardware. However, in this case, a large circuit is required and it is difficult to make the size smaller when it is integrated as IC (integrated circuit or semiconductor device). Especially, when it is used in the mobile devices such as a cellular phone, it can not meet a request for downsizing of the device.
On the other hand, all the process of the encoding may also be carried out by software. However, in this case, a load of a Central Processing Unit (CPU) which processes the software will be increased. Therefore, a time which the CPU can spend for processing other things will be limited and a performance of equipment in which the CPU is mounted will be lowered. In addition, a processing time of the CPU will get longer and a large amount of power will be consumed. Especially, when it is used in the mobile devices such as the cellular phone, it can not meet a request for reducing the power consumption in order to curtail the draining of a battery.
Given these factors, all the process of the encoding may be carried out by the hardware and the software. However, after consideration, it turned out that the rate control method disclosed in MPEG-4 Visual Part (recommendation report ISO/IEC 14496-2: 1999(E) Annex L) cannot be carried out when allocation of the hardware and the software is optimized. In other words, when the process is allocated between the hardware process and the software process which have different processing speeds, a buffer is needed in order to absorb a difference of the processing speed. But when the buffer is provided, the above-mentioned rate control method cannot be carried out. This means that it is difficult to satisfy both of the allocating optimization of the hardware and the software and the production of the compressed data at the regular rate in the compression process of the image data.
Moreover, even if the production rate of the compressed data can be controlled, a block-noise often appears in the image formed of the expanded data with the rate control method described in MPEG-4 Visual Part (recommendation report ISO/IEC 14496-2: 1999(E) Annex L). Therefore, there is still a problem that an image quality could be deteriorated.
Furthermore, when the amount of the compressed data is increased in order to prevent the image quality from being deteriorated, a desired rate at which the data is sent cannot be maintained.
The present invention has been developed in consideration of the above-mentioned problems, and intended to provide an image data compression device which can steadily generate a compressed data at a regular rate while optimizing allocation of an image data compression process to hardware and software and preventing the image quality from being deteriorated at the same time. The present invention also is intended to provide an encoder, electronic equipment and a method of compressing image data.